In the past, portable heat packs have found many uses including, but not limited to, providing heat for food preparation, medical treatments, and comfort in remote locations. For food preparation, the portable heating pack should be able to cook the food relatively quickly, while not overcooking or undercooking the food. The heating mechanism utilized by the portable heating pack should also be relatively cheap, environmentally and consumer-wise safe, and be fail-proof. The portable heating packs are typically self contained, self heated packs which provide heat from exothermic reactions carried out within the heating pack. In many cases the portable heating packs utilize a water reactive chemical compound which produces heat via a hydrolysis reaction upon contacting water. Other portable heating packs have used chemical/electrochemical reactions such as corrosion of a metal such as magnesium or iron to produce heat.
U.S. Pat. Nos. 3,079,911 and 5,205,277 discloses the use of calcium oxide (quick lime) in a portable heating pack. Water is added to the portable heating pack and reacts with the calcium oxide to form calcium hydroxide and heat. The drawback of this type of system is that the heat of hydrolysis of the calcium oxide can be large and somewhat uncontrollable to be useful.
The corrosion of a magnesium/iron alloy has worked particularly well for use in portable heating packs and has been the subject of multiple U.S. patents. U.S. Pat. Nos. 4,522,190, 5,220,909, and 5,355,869 disclose the use of a supercorroding alloy of magnesium and iron in a portable heating device, termed “Flameless Ration Heaters” (FRH), used for heating food. FRH devices are widely produced and utilized each year. A typical FRH weighs approximately ½ ounce and can raise the temperature of an 8 ounce entree 100° F. in 10 minutes. The FRH has been found superior to other types of portable heating packs in regards to cost, safety and performance, however, during operation each FRH releases approximately 8 liters of hydrogen which is vented to the atmosphere.
In the FRH, a magnesium/iron alloy is brought into contact with water or an ionically conducting solution, which corrodes the alloy thereby producing heat. The reaction of magnesium and water is relatively slow due to the formation of an oxide/hydroxide coating on the surface of the magnesium, however, the addition of iron to the magnesium has been found to accelerate the reaction. The amount of iron can be varied to control the rate of reaction thereby controlling the amount of heat produced. The reaction between magnesium and water is actually an electrochemical reaction where the anodic corrosion of magnesium is supported by the cathodic hydrogen evolution taking place on the iron. While this reaction works particularly well for providing a controlled amount of heat, a side product of the reaction is flammable and potentially explosive hydrogen gas. During normal use the amount of hydrogen produced may not reach the lower explosive limit (LEL) for hydrogen in air, but in confined quarters where one or more of the portable heating devices are used simultaneously, lower explosive limits (LEL) may be reached. By minimizing or safely collecting and storing the hydrogen produced by the FRH, the safety of the FRH will be improved thus making the FRH suitable for use in confined quarters and additional applications.